ARM: dts: imx6ull-colibri: change hierarchy of DTS files

Introduce imx6ull-colibri-u-boot.dtsi for u-boot specific properties to
keep original imx6ull-colibri.dts in sync with Linux.

Move all contents of imx6ull-colibri.dts to imx6ull-colibri.dtsi +
additionally fix checkpatch warnings.

Reviewed-by: Oleksandr Suvorov <oleksandr.suvorov@toradex.com>
Reviewed-by: Fabio Estevam <festevam@gmail.com>
Signed-off-by: Igor Opaniuk <igor.opaniuk@toradex.com>